Generated by the body, the auditory or the nervous system, with no external stimulus. However, the theory that the Hum is actually tinnitus fails to explain why the Hum can be heard only at certain geographical locations, to the degree those reports are accurate. There may exist individual differences as to the threshold of perception of acoustic or non-acoustic stimuli, or other normal individual variations that could contribute to the perception of the Hum by some people in the population and not by others.

While the Hum is hypothesized to be a form of low frequency tinnitus[12] such as the venous hum, some sufferers claim it is not internal, being worse inside their homes than outside. However, others insist that it is equally bad indoors and outdoors. More mystery is added as some notice the Hum only at home, while others hear it everywhere they go. Some reports indicate that it is made worse by attempted soundproofing (e.g., double glazing), which serves only to decrease other environmental noise, thus making the Hum more apparent. Tinnitus, as well, is generally worse in places with less exterior sound.

People who both suffer from tinnitus and hear the Hum describe them as qualitatively different, and many hum sufferers can find locations where they do not hear the hum at all. An investigation by a team of scientists in Taos dismissed the possibility that the Hum was tinnitus as highly unlikely.Copyright Source: [link to en.wikipedia.org]